S.K. Hoffmann

Weak long-distance superexchange interaction and its temperature variations in copper(II) compounds studied by single crystal EPR

In this paper we review experimental EPR data and theoretical models of the weak super-exchange interaction (J<0.1 cm−1) mainly in copper(II) compounds single crystals. The different EPR techniques are described which allow one to determine the exchange coupling such weak asJ=0.0001 cm−1. Such a weak interaction is detectable by EPR on distances up to 25 Å and the presented data allowed us to establish empirical limits for a weak exchange coupling transmission forJ<0.3 cm−1 as |Jlim|=5.9exp(−0.335R) whereR (Å) is the intermolecular distance. The weak superexchange coupling was found to be temperature dependent and this dependence is very specific to a crystal or molecular structure. In most cases temperature dependence of superexchange is governed by thermal crystal lattice contraction. A phenomenological model of competing potential and kinetic exchange is presented and used for description of copper(II) compounds with opposite temperature behaviour of superexchange coupling. Possible mechanisms driving temperature variations of superexchange are discussed.

Crystal field theory and EPR parameters in D2d and C2v distorted tetrahedral copper(II) complexes

Abstract Calculations of the orbital energy vs tetrahedral ( D 2 d and C 2 v ) distortion parameters are reported for copper complexes on the assumption of constant metal-ligand distance. The possible ground states of the complexes are considered and the respective spin Hamiltonian parameters vs distortion parameters dependences are calculated. Comparison with experiment led to good agreement: the relation between the orbital splittings and dihedral distortion angle is found to be linear for symmetry C 2 v in a wide range of distortion of the regular tetrahedron, and distortion of the D 2 d crystal field has a compensatory effect lowering the hfs splitting from 4 p -orbital admixture into the ground-state wavefunction.

Water and lipid relations in beech (Fagus sylvatica L.) seeds and its effect on storage behaviour

Beech (Fagus sylvatica L.) seeds indicate intermediate storage behaviour. Properties of water in seed tissues were studied to understand their requirements during storage conditions. Water sorption isotherms showed that at the same relative humidity (RH) the water content is significantly higher in embryo axes than cotyledons. This tendency maintains also after recalculating the water content for zero amount of lipids in tissues. Differential thermal analysis (DTA) indicated water crystallization exotherms in the embryo axes at moisture content (MC) higher than 29% and 16% in the cotyledons. In order to examine the occurrence of glassy state in the cytoplasm of beech embryos as a function of water content, isolated embryo axes were examined using electron spin resonance (ESR) of nitroxide TEMPO probe located inside axes cells. TEMPO molecules undergo fast reorientations with correlation time varied from 2 x 10(-9) s at 180 K to 2 x 10(-11) s at 315 K. Although the TEMPO molecules label mainly the lipid bilayers of cell membranes, they are sensitive to the dynamics and phase transformation of the cytoplasmic cell interior. The label motion is clearly affected by a transition between liquid and glassy state of the cytoplasm. The glass transition temperature (T(g)) raises from 253 to 293 K when water content decreases from 18% to 8%. Far from T(g) the motion is described by Arrhenius equation with very small activation energy E(a) in the liquid state and is relatively small in the glassy state where E(a)=1.5 kJ/mol for 28% H(2)O and E(a)=4.7 kJ/mol for 8% H(2)O or less. The optimal storage conditions of beech seeds are proposed in the range from 255 K for 15% H(2)O to 280 K for 9% H(2)O.

Structure and dynamics of S3− radicals in ultramarine-type pigment based on zeolite A: Electron spin resonance and electron spin echo studies

X-band electron spin resonance (ESR) spectra of S(3)(-) radicals in ultramarine analog (pigment) prepared from zeolite A and maintaining the original structure of parent zeolite were recorded in the temperature range of 4.2-380 K. Electron spin echo experiments (echo detected ESR, electron spin-lattice relaxation, and spin echo dephasing) were performed in the temperature range of 4.2-50 K. The rigid lattice g factors are g(x) = 2.0016, g(y) = 2.0505, and g(z) = 2.0355, and they are gradually averaged with temperature to the final collapse into a single line with g = 2.028 above 300 K. This is due to reorientations of S(3)(-) molecule between 12 possible orientations in the sodalite cage through the energy barrier of 2.4 kJ/mol. The low-lying orbital states of the open form of S(3)(-) molecule having C(2v) symmetry are considered and molecular orbital (MO) theory of the g factors is presented. The orbital mixing coefficients were calculated from experimental g factors and available theoretical orbital splitting. They indicate that the unpaired electron spin density in the ground state is localized mainly (about 50%) on the central sulfur atom of S(3)(-) anion radical, whereas in the excited electronic state the density is localized mainly on the lateral sulfur atoms (90%). A strong broadening of the ESR lines in directions around the twofold symmetry axis of the radical S(3)(-) molecule (z-axis) is discovered below 10 K. It is due to a distribution of the S-S-S bond angle value influencing mainly the energy of the (2)B(2)-symmetry MO. This effect is smeared out by molecular dynamics at higher temperatures. A distribution of the g factors is confirmed by the recovery of the spin system magnetization during spin-lattice relaxation measurements, which is described by a stretched exponential function. Both the spin-lattice relaxation and electron spin echo dephasing are governed by localized phonon mode of energy of about 40 cm(-1). Thus, the anion-radical S(3)(-) molecules are weakly bonded to the zeolite framework, and they do not participate in the phonon motion of the host lattice because of their own local dynamics.

Electronic structure, Jahn-Teller dynamics and electron spin relaxation of two types of octahedral Cu(II) complexes in cadmium formate dihydrate Cd(HCOO)2.2H2O. EPR and ESE studies

Cd(HCOO)2·2H2O single crystals weakly doped with Cu(II) ions have been studied by cw-EPR (4.2–300 K) and by electron spin echo – ESE (4.2–60 K). Copper(II) ions substitute Cd(II) in two different sites forming Cu(HCOO)6–Cuf and Cu(HCOO)2(H2O)4–Cuw octahedral complexes with strong preference to Cuf as shown by the intensity ratio of the EPR spectra (up to 20∶1). Despite different molecular structures both complexes have nearly identical EPR parameters at rigid lattice limit with gz=2.429, gy=2.092, gx=2.064, Az=120, Ay=32 and Ax=12×10−4 cm−1 for Cuf. This fact as well as strong axial deformation of the crystal field at Cu(II) sites indicate that the strong Jahn–Teller effect operates producing three wells in the potential surface with one having much lower energy than the others. In the Cuf complex the dynamic J–T-effect has been observed as a vibronic averaging of the two g and A parameters (along z and y axes). It indicates that only one of the higher energy wells is thermally accessible and the Silver–Getz model leads to the average energy difference between the two lowest energy wells δ12=500(60) cm−1. The δ12 is temperature dependent. For Cuw complex no vibronic effects were observed in EPR spectra indicating that higher energy wells are not populated up to 300 K. The spin–lattice relaxation time T1 and phase memory time TM were measured up to 60 K only, because for higher temperatures the ESE decay was too fast. Spin–lattice relaxation is governed by two-phonon Raman processes which allow one to determine the Debye temperature of the crystal as ΘD=193 K. The ESE decay was described as V(2τ)=V0exp(−τ/b−mτ2) indicating the contribution of the spectral diffusion (quadratic term). The ESE dephasing rate 1/TM is governed by spectral diffusion below 15 K. For higher temperatures the T1-processes and excitations to the higher vibronic levels of energy Δ=166 cm−1 give comparable contributions.

Raman spin-lattice relaxation, Debye temperature and disorder effects studied with electron spin echo of Cu2+ in Tutton salt crystals

Spin-lattice relaxation time T1 was determined by the electron spin echo (ESE) method in the temperature range 4-60 K in a series of Tutton salt crystals MI2MII(SO4)26X2O (MI = NH4, K; MII = Zn, Mg; X = H, D) weakly doped (≤1018 ions cm-3) with the 63Cu2+ isotope. The ESE signal was undetectable at higher temperatures. The relaxation rate increases over the six decades in the studied temperature range with T1 equal to 1 s at 4 K and 0.5 µs at 50 K. Various possible relaxation mechanisms are discussed with the conclusion that the relaxation is governed by two-phonon Raman processes without a noticeable contribution from the reorientations of Cu(H2O)6 octahedra between Jahn-Teller distorted configurations. Deuteration of the crystal has no effect in spin-lattice relaxation. For a few crystals, having the largest Cu2+ concentration among the studied crystals, a strong and linear in temperature contribution to the relaxation rate was found below 15 K. Possible explanations are discussed with the final conclusion that this effect is due to a non-uniform Cu2+ distribution in the host lattice producing effective relaxation via pairs and triads of the Cu2+ ions. From the T1(T) dependence the Debye temperature ΘD was determined for the all crystals studied. This varies from ΘD = 166 K for K2Zn(SO4)26H2O to ΘD = 238 K for (NH4)2Mg(SO4)26H2O. The ΘD values are discussed and used for calculation of the sound velocity which was found to be similar in all crystals and equal to ν = 4150(±150) m s-1.

Electron spin echo and EPR studies of paramagnetic centers in polycrystalline C60

Abstract Three different paramagnetic centers were detected by X-band EPR in C 60 powder sample below 60 K and identified as C 60 + radical ( g =2.0028), dimers of C 60 + radicals with zero field splitting D =0.00054 cm −1 , and conducting electrons ( g =2.0021) of graphite residuals. Only C 60 + EPR signals persist at room temperature and all spectral lines saturate easily for microwave powers higher than 50 mW. Two spin-lattice relaxation time T 1 and two phase memory times T M describe the system with T 1 =500 μ s and T M =2.6 μ s for C 60 + at 20 K. An anomaly in T 1 and T M is observed at about 55 K where the field-swept ESE spectrum is narrowest and where the anomaly in the dielectric permittivity is observed. It indicates an existence of a phase transition in this temperature range.

Electron spin echo studies of spin-lattice and spin-spin relaxation of SeO3- radicals in (NH4)3H(SeO4)2 crystal

Abstract The spin-lattice and spin-spin relaxation of SeO 3 - radical in (NH 4 ) 3 H(SeO 4 ) 2 single crystal was investigated using the two-pulse electron spin echo method. Spin-lattice relaxation time T 1 varies from 1.7 ms at 5 K to 0.6 μs at room temperature and is governed by direct relaxation process below 40 K and by two-phonon Raman process at higher temperatures. The phase memory time T M varies from 178 ns to 546 ns with the minimal value at about 50 K. T M -value is not related to the EPR linewidth and is determined by a radical motion below 100 K and by spin-lattice relaxation above 100 K.

EPR and ESE of CuS4 complex in Cu(dmit)2: g-Factor and hyperfine splitting correlation in tetrahedral Cu–sulfur complexes

Pseudotetrahedral CuS4 complexes of Cu(dmit)2 compound in DMF solution were studied by EPR, UV-Vis and electron spin echo methods. After rapid freezing at 77 K a good glassy state is formed and the CuS4 complex has a D(2d) symmetry of a compressed tetrahedron with xy ground state and spin-Hamiltonian parameters g||=2.089, g⊥=2.026, A||=146×10(-4) cm(-1) and A⊥=30×10(-4) cm(-1). The complex is not deformed in the glassy state and is very rigid as indicated by the echo detected spectrum and by electron spin relaxation which is governed by reorientations of methyl groups of surrounding DMF molecules as shown by electron spin echo envelope modulation (ESEEM) spectrum. The g|| and A|| of Cu(dmit)2 and other CuS4 complexes collected in Peisach-Blumberg correlation diagram were analyzed using extended Molecular Orbital theory. We explain why the correlation line for copper-sulfur complexes has larger slope compared to the CuO4 and CuN4 tetrahedra. Along the correlation line the delocalization of unpaired electron density onto ligand is constant and varies from β=0.78-0.83 for g|| in the range 2.06-2.10 of correlation diagram. The slope of the line is determined by the product of MO-coefficients αc1, where α is a parameter characterizing delocalization of unpaired electron in x(2)-y(2) and c1<1 is a mixing parameter decreasing when 4p contribution grows. We found, unexpectedly, that αc1≈0.7 for all CuS4 complexes suggesting a correlation between degree of tetrahedral deformation and MO-parameters. MO-coefficients for Cu(dmit)2 are α=0.753, β=0.752 and c1=0.930 confirming a strong delocalization of unpaired electron in xy and x(2)-y(2) orbitals.

Electronic structure and dynamics of low symmetry Cu2+ complexes in kainite-type crystal KZnClSO4·3H2O: EPR and ESE studies

EPR measurements at X-band were performed in the temperature range 4.2-300 K with angular dependence measurements at 77 K for Cu(2+) in KZnClSO(4).3H(2)O. Rigid lattice spin-Hamiltonian parameters are: g(z) = 2.4247, g(y) = 2.0331, g(x) = 2.1535, A(z) = -103 x 10(-4) cm(-1), 63 x 10(-4) cm(-1), and -31 x 10(-4) cm(-1). The parameters were analyzed using MO-theory with the d(x(2)-y(2)) ground state containing admixture of the d(z(2))-state in the rhombic symmetry D(2h). The analysis consistently explained unusual g-factor sequence and relatively small hyperfine splitting anisotropy as the consequence of the mixing and spin density delocalization via excited orbital states. We assigned that Cu(2+) ions substituting host Zn(2+) prefer one of the four structurally different zinc sites where they are coordinated by four water molecules and two SO(4) groups in an distorted octahedron elongated along SO(4)-Cu-SO(4) direction. The distortion is due to the Jahn-Teller effect which is static at low temperatures but becomes dynamic above 20 K with jumps of the Cu(2+) complex between two lowest potential wells. The jumps produce continuous g-factor and hyperfine splitting averaging when temperature increases. This process is discussed in terms of two motional averaging theories: classical theory based on generalized Bloch equations and Silver-Getz model. Their limitations are discussed. Importance of the difference in the g-factors of the averaged line is explained and a new expression for calculation of jump frequency from the line shift is proposed. The jumps are described as phonon induced tunneling via excited vibrational level of energy 76 (+/-6) cm(-1). This process is not effective enough at low temperatures and Boltzmann population of the two lowest energy potential wells is reached above 110 K. From electron spin-lattice relaxation measurements by electron spin echo methods the Debye temperature was determined as Theta(D) = 172 K. Fourier Transform of strongly modulated spin echo decay gives pseudo-ENDOR spectrum with peaks from (1)H and (35)Cl nuclei. From splitting of the peaks into doublets we determined the distance to the modulating nuclei and confirmed the position of the site where Cu(2+) ion is located.